Membranes may be defined as thin, solid materials that permit the selective transport of certain chemical species over others.
Silica membranes prepared by chemical vapor deposition (CVD) or sol-gel methods on mesoporous supports are effective for selective hydrogen permeation [T. Okubo and H. Inoue, J. Membr. Sci., 42 (1989) 109; G. R Gavalas, et al., Chem. Eng. Sci., 44 (1989) 1829; S. Yan et al., Ind. Eng. Chem. Res., 33 (1994) 2096]. However, it is known that hydrogen-selective silica materials are not hydrothermally stable. Most researchers have reported a loss of permeability of silica membranes (as much as 95% or greater in the first 12 h) on exposure to moisture at high temperature. Sea et al. found that the hydrogen permeance of a silica membrane deposited on mesoporous γ-alumina substrates was decreased by 90% from 3.5×10−7 to 4.0×10−8 mol m−2s−1Pa−1 after exposure to 50 mol % water vapor at 400° C. for 100 h [Sea et al. Ind. Eng. Chem. Res. 37 (1998) 2502]. Wu et al. reported a decrease of 62% and 70% in the permeances of He and N2 for a CVD-deposited silica membrane treated at 600° C. under a N2 flow containing 20 mol % water vapor [Wu et al., J. Membr. Sci., 96 (1994) 257]. This is because the porous silica (SiO2) easily undergoes densification upon exposure to water vapor at elevated temperatures. The densification involves the formation of Si—O—Si bonds from silanol groups (Si—OH) catalyzed by water, leading to the shrinkage of pores (Iler, The Chemistry of Silica, Wiley, New York, 1979).
Much effort has been expended on the improvement of the stability of silica membranes. One approach is to make hydrophobic silica membranes prepared by the incorporation of methyl groups in the silica microstructure [de Vos et al., J. Membr. Sci., 158 (1999) 277].], [Y. S. Lin, I. Kumakiri, B. N. Nair, H. Alsyouri, “Microporous Inorganic Membranes”, Separ. Purif. Methods, 2002, 31, 229-379].
On the other hand, composite membranes prepared by sol-gel methods composed of silica with other inorganic oxides such as alumina (Al2O3), titania (TiO2) and zirconia (ZrO2) have been reported as better alternatives to silica membranes for use under humid atmospheres at high temperature. Fotou et al. [Fotou et al., J. Mater. Sci., 30 (1995) 2803] introduced these oxides and MgO into the membranes by doping the starting silica sol with controlled amounts of the corresponding nitrate salts. They found that the mean pore size did not change much from 0.6 nm to 0.7 nm, but the hydrothermal stability was improved after doping with 3% alumina. A heat treatment in 50 mol % steam/air at 600° C. for 30 h resulted in 63.6% reduction in the surface area and a loss of 86.5% micropore volume for the unsupported 3% alumina-doped silica membrane, compared to 84.6% and 94.5%, respectively for a pure silica membrane. They also reported that 6% alumina-doped and magnesia-doped silica membranes were not improved, since the surface area was substantially reduced compared with the pure silica. The membranes prepared by Fotou et al. differ from our membranes because they consist of layers of sol particles without a continuous top layer deposited by CVD. As such they have spaces in between the particles that give rise to poor selectivity. The authors do not report permeance or selectivity, however, these properties have been measured for similar membranes. The permeation properties of such membranes prepared by the sol-gel deposition of SiO2-10 mol % Al2O3 and SiO2-10 mol % TiO2 compositions on a gamma-alumina support were reported by Hekkink et al. [Hekkink et al., Key Eng. Mater., 61&62 (1991) 375]. The H2 permeances at 298 K were 7×10−7, 2.2×10−7 and 6×10−8 mol m−2s−1 Pa−1 for pure SiO2, SiO2-10 mol % TiO2 and SiO2-10 mol % Al2O3 membranes, respectively. The SiO2—Al2O3 derived membrane had permeance for H2 of 2.5×10−7 mol m−2s−1Pa−1 at 301 K and the SiO2—TiO2 derived membrane had permeance for H2 of 6.7×10−7 mol m−2s−1Pa−1 at 473 K. These permeances are high, but the selectivities over CO were only 3 and 9. This is indicative of the presence of channels that permit passage of all gases, and is typical for membranes prepared by the sol-gel method. Selectivity can be increased, but only at the cost of reducing permeance, as discussed below.
As another example of work on sol-gel membranes, Yoshida et al. investigated the hydrothermal stability of sol-gel derived silica-zirconia membranes with a content of zirconia of 10-50 mol % [Yoshida et al., J. Chem. Eng. Japan, 34 (2001) 523]. After a 20 h-exposure to a high temperature of 773 K and steam at levels of 13-33 mol %, a SiO2-10 mol % ZrO2 membrane still suffered a decrease of H2 permeance of 70% to 8.9×10−8 mol m−2s−1Pa−1, but with an increased selectivity of H2 to N2 of 190, while the SiO2-50 mol % ZrO2 membrane did not show any change in the H2 permeance but a constant H2/N2 selectivity of 4.0. It is well known in the membrane field that selectivity can be increased with a drop in permeance, and this represents a typical example of this phenomenon.
The composite membranes prepared by the chemical vapor deposition method generally have a better selectivity but a lower permeance in comparison to those obtained with the sol-gel procedure. Nam et al. made SiO2—TiO2 membranes at 873 K on porous Vycor glass with a mean pore diameter of 4 nm by hydrolysis of tetraisopropyl titanate (TIPT) and tetraethyl orthosilicate (TEOS) at atmospheric pressure [Nam et al., Korean Membr. J., 3 (2001) 69]. Using molar ratios of TIPT/TEOS in the range of 0.1-7, composite membranes were obtained that showed high selectivities of around 500 but with low permeance of 2×10−8 mol m−2s−1Pa−1 at 873 K. This again is an example of the tradeoff between selectivity and permeance that gives rise to high selectivity at the cost of low permeance.
To obtain ceramic membranes with both high selectivity and permeance, some researchers have used mesoporous or macroporous supports with pore diameters larger than 50 nm to decrease the resistance of the supports. By using an intermediate mesoporous gamma-alumina layer, Yan et al. and placing the silica in the pores of the support by CVD they obtained a H2 permeance of 1.8×10−1 mol m−2s−1Pa−1 but a selectivity of H2 to N2 of only 26 at 873 K [Yan et al. Ind. Eng. Chem. Res., 33 (1994) 2096]. The poor selectivity was due to the presence of defects due to the use of an intermediate layer of large particle size, which left large openings between the particles. Oyama et al. made membranes with the silica layer on the outer surface of an alumina substrate and obtained permeability of 2.2×10−7 mol m−2s−1Pa−1 and a selectivity of H2 to CO of 370 at 873 K [S. T. Oyama, L. Zhang, D. Lee, D. S. Jack, “Hydrogen Selective Silica-Based Membranes” U.S. Pat. No. 6,854,602 B2, Feb. 15, 2005]. Recently, we have successfully prepared a gamma-alumina multilayer with a graded structure by sequentially placing boehmite sols of gradually decreasing particle sizes on a macroporous alumina support Oyama et al. [S. T. Oyama, Y. Gu, D. Lee, U.S. patent application Ser. No. 10/775,288, Feb. 10, 2004,]. The multilayer graded structure had a thickness of tens of nanometers and was substantially defect-free. After deposition of a thin silica layer by the CVD technique method described in a patent [Hydrogen-Selective Silica Based Membrane, S. T. Oyama, A. Prabhu, U.S. Pat. No. 6,527,833, Mar. 4, 2003], the resulting silica-on-alumina membranes had excellent permeability of 3.0×10−7 mol m−2s−1 Pa−1 and good selectivity for hydrogen over CH4, CO and CO2 of over 500 at 873 K.